Just walking at São Paulo is already hard for me because of my poor red blood cells count. At 6,500ft things get a little bit harder.
For you to get it, in normal shape/ health, the altitude gain I needed to do for the summit of this mountain (450m) would be covered in just over one hour back in the day (the trail is 5kms long and very easy).
This time, it took me five hours to do it, headaches, some nausea...
It feels like 20.000 feet, simple as that.